Superheterodyne circuit



Jam., M, m..

w.I VAN B. ROBERTS SUPERHETERODYNE CIRCUI T Filed Jan. ll, 1930 NVENTOR WALTER VAN BROBERTS ATTORN EY Patented Jan. 14, 1936 UNITED STATES SUPERHETERODYNE CIRCUIT' Walter van B. Roberts, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application January 11, 1930, Serial No. 420,125

12 Claims. (Cl. Z50-20) In superheterodynereceiving circuits the use of a high intermediate frequency insures that for a given oscillator setting only one station in the broadcast range can be received. In such cir- -.cuits interference from frequencies outside the broadcast range may be eliminated by means of fixed low pass filter circuits. Thus complete selectivity can be obtained by this method without the use of any variable element other than the heterodyne oscillator frequency. Unfortunately, when the ordinary means for heterodyning are used the system as above described has a certain defect which may best be explained by considering a specific problem. Suppose the intermediate frequency chosen is 2000 kilo-cycles and a station of 1000 kilo-cycles is impressed on the grid of a frequency changing tube of the ordinary type. The assymetric characteristic of the tube, which, of course, exists in order to allow frequency changing, also necessarily allows the 1000 kilo-cycle input to produce double frequency in the output circuit which double frequency, in this particular instance, is the same as the chosen intermediate frequency. It is evident therefore that this particular station could be received without the use of any heterodyne, and if heterodyne is employed, and not adjusted to give absolutely accurately the same beat frequency as the double frequency of the station, beats will be produced and heard between the two frequencies each of which is close enough to 2000 kilocycles to pass the selective intermediate frequency amplier. The same sort of trouble may occur on stations having 1/4 or 1/3 the frequency of the `intermediate amplifier. The reason for the above mentioned defect seems to be that frequency changers usually used will allow the production of a beat frequency when the heterodyne and the signal are applied in series in the same circuit. A device having two separate circuits such that the signal must be applied to one of them and the heterodyne to the other in order to produce beats would not be subject to this difficulty. To this end I make use of a phenomenon described in the proceedings of the Institute of Radio Engineers, volume 6, page 5, February 1918 by A. lW. Hull, entitled The Dynatron, a Vacuum Tube Possessing Negative Resistance, in which report there was described a circuit employing a tube similar in structure to what is known in the art as a shielded plate or screen grid tube.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims.

My invention itself, however, both as to its organization and method of operation will best be understood by reference to the following description taken invconnection with the accompanyingf drawing, in` which:

Figure 1-illustrates diagrammatically a circuit which may be utilized for obtaining certain characteristics of the space discharge device employed;

Figures 2 and 3 illustrate curves obtained by varying certain constants in the circuit shown 5 diagrammatically by Figure l.;

Figure 4 illustrates diagrammatically a preferred embodiment of my invention; and,

Figure 5 illustrates a modification of my invention. j 10 To demonstrate the action made use of in my invention reference is had to Figure 1 wherein there is shown an electronic tube l of the screen grid type provided with cathode energizing source 5, biasing source 4 and anode potential and screen 15 grid potential supply 6. As shown, the anode potential is variable and is connected in series in the output circuit of the tube with a current meter 2. The biasing potential supply 4 is also shown variable for reasons which will appear 20 hereinafter. In said figure if we consider that all potentials supplied by sources 5 and 6 are fixed except that of the plate which is adapted to be varied as shown and the resulting plate current as given by meter 2 is plotted (see Figure 2) 25 we would with a comparatively low value of screen potential obtain a curve such as shown by the solid line A of Fig. 2. Now supposing that the screen potential of a higher value is taken from source 6 (Fig. 1), in such a case a curve will be 30 obtained as shown by the dotted line B. In both instances the grid potential supplied by source 4 is the same. The falling characteristic is probably due to secondary emission and is a well known phenomena. By properly choosing the 35 screen potential, curves may be obtained as shown in Figure 3 where the plate current variesalmost linearly through zero. The curve obtained, as will be noted, is quite symmetrical on both sides of the axis for a considerable distance. In Fig- 40 ure 3 the broken curve C is for a slightly negative grid bias, the dotted curve D for a more negative grid bias and the solid curve for a still greater negative bias. The important thing to be noted in Figure 3 is the fact that whatever grid bias 45 is taken, the plate current passes through zero at the same Value of plate potential. Thus by proper choice of plate potential and screen potential the tube may be operated under such conditions that the plate current Will be zero forV all 50 values of grid'potential.

Referring now to Fig. 4, there is shown a tube 3 which is of the same type as the tube shown in Fig. 1 provided with an input circuit including a source of biasing energy 8 and the secondary of 55 a transformer `l I. The primary of the transformer il is connected to a suitable source of signal energy. The customary source of anode energizing potential issupplied through a source 5 as shown. The output circuit of the tube includes 60 the secondary of a transformer I0, the primary of a transformer I2 and a source of anode potential 'I all in series connected between the anode and cathode of tube 3. The screen grid electrode is connected to a point of said output circuit between the source 1 and the primary of transformer I2 through a suitable screen grid potential source 9. The primary of the transformer I0 is connected to a source of heterodyne energy, preferably a source of locally generated oscillations of suitable frequency. The secondary of the transformer I 2 is connected in suitable manner to an intermediate frequency amplifier as indicated in the drawing. It will be noted that in the arrangement shown in Fig. 4 the potential applied to the screen grid electrode as respects the cathode is higher than the potential applied to the anode with respect to the cathode.

In Figure 4 supposing that the plate potential supplied by source 'I of space discharge device 3 is chosen as above described, that is, so that the plate current is Zero irrespective of grid bias as shown in Figure 3, then since the plate current is zero for all values of grid potential it will obviously be impossible to obtain double frequencies or beats by impressing signals alone or signals plus heterodyne voltages on the grid circuit only. Hence, if the station referred to above as 1000 kilo-cycles is being received no intermediate frequency of 2000 will be produced by the station alone, or signals plus heterodyne voltages on the grid circuit only. The action may be analyzed as follows: The slope of the curves of Figure 3 where they cut the axis varies directly with grid bias as was shown in the above referred to discussion. This slope, however, is a measure of the conductance of the plate circuit to the heterodyne voltage impressed. The plate current being the product of the heterodyne voltage and the plate conductance is thus proportional to the product of the heterodyne and signal voltages but does not contain any term in higher powers of the signal voltage alone. The product term represents the two beat frequencies produced, one of which is selected by the intermediate frequency system.

In some instances it is desirable to avoid the use of a separate oscillator for obtaining the desired beat frequency. To satisfy these conditions the circuit shown in Fig. 5 is provided. In said figure there is shown what may be said to be a simple working embodiment of the invention which differs from Fig. 4 in that the heterodyne energy is not produced by a separate oscillator but by inserting a tuned circuit TC composed of inductance coil I3 and tuning condenser I4 in the plate circuit of tube 3. VThis tuned circuit oscillates by virtue of the following characteristic of the plate current. Since the various other portions of Fig. 5 are substantially like those described above in connection with Fig. 4

no detailed description of Fig. 5 will be given except to state that the input of the tube 3 is connected to an antenna I5 through a coupling element I9. It should be noted that the input of the tube 3 is tuned by means of the variable tuning element 20.

It is to be understood, of course, that the invention is not limited to the particular applications illustrated and that the numerical values given in the description are only by way of eX- ample.

Having thus described my invention what I desire to protect by Letters Patent is as follows:

1. In a signal receiving system a space discharge device having an input circuit and an output circuit and comprising a cathode, a grid electrode, a shielding electrode and an anode, means for impressing signal energy on said input circuit, a source of direct current potential for said shielding electrode, a source of direct current potential for said anode, the direct current potentials applied to said shielding electrode and said anode being normally so related that the conductivity of said output circuit is approximately zero irrespective of energy impressed upon said input circuit and means for periodically varying the anode potential.

2. In a signal receiving system, a space discharge device having an input and an output circuit, and comprising a cathode, a grid electrode, a shielding electrode and an anode, a source of high frequency oscillations, means for impressing said oscillations on said input circuit, a direct current potential source for said shielding electrode and said anode, the value of the direct current potentials applied to said shielding electrode and anode being normally so related that the anode circuit is maintained approximately non-conducting irrespective of energy impressed upon said input circuit, and means comprising a second source of oscillations coupled to said output circuit, for varying the direct current potential applied to said anode.

3. 4In a receiving system of the superheterodyne 'type a frequency changer comprising,l a space discharge device, input and output circuits therefor, means for impressing signal frequency energy in one of said circuits, means for impressing energy of another frequency in the other of said circuits, means for operating said device at a point of its plate-current, plate-potential characteristic wherein the plate current will be zero for all values of grid potential whereby the output current of the device contains no term proportional to the powers of one of said energies when impressed in the absence of the other, the output current of said device in the presence of both said energies being proportional to the product or the two energies impressed in said circuits.

4. In a receiver, a dynatron circuit including a space discharge device provided with an anode, a cathode, a signal grid and an auxiliary electrode, means for maintaining the anode and auxiliary electrode at a positive potential with respect to the cathode and means for maintaining the signal grid negative with respect to the cathode; the relationship between the respective values of the potentials applied to the anode and auxiliary electrode are such that the anode circuit will be substantially non-conductive irrespective of the value of the energy impressed upon the input circuit and means for periodically varying the plate potential a predetermined amount above and below said above mentioned value said last-named means comprising a source of oscillations.

5. In a superheterodyne receiver, a dynatron circuit comprising an electronic tube provided with at least an anode, a cathode, a signal grid and an auxiliary grid, means for maintaining both the auxiliary grid and the anode at a positive potential with respect to the cathode, the auxiliary grid being maintained at a substantially higher positive potential than the anode, circuit connections between the elements of the tube comprising a grid-cathode circuit, an anodecathode circuit, and an anode-screen-grid circuit, means for impressing signal energy upon said grid-cathode circuit, means for varying the negative conductance of said anode-cathode circuit in accordance with said signal energy, a source of heterodyning frequency energy coupled to said anode-cathode circuit adapted to impress heterodyning frequency energy thereon, said signal energy and heterodyning energy being combined to produce energy of an intermediate frequency and an intermediate frequency ampliiier circuit coupled to said anode-screen-grid circuit.

6. In a superheterodyne receiver, a dynatron frequency changer circuit having an input and an output in which the relationship between the respective values of the potentials applied to the anode and fourth electrode of the dynatron are such that the anode circuit will be substantially non-conductive irrespective of the value of the grid potential, means for impressing signal energy upon said input, a source of heterodyning frequency energy coupled to said output for periodically varying the plate potential and an intermediate frequency utilizing circuit coupled to said output.

'7. In a superheterodyne receiving system, a frequency changing means comprising a space discharge device having an input circuit and an output circuit, means for impressing signal frequency energy in said input circuit, means for impressing energy of another frequency in the output circuit, and means for having an output current in said output circuit proportional to the product of the two energies impressed in said input and output circuits, said space discharge device being conditioned so that it operates in the vicinity of that point of its plate current-plate potential characteristic curve at which the output current contains no term proportional to the powers of the energy impressed upon said input circuit when impressed in the absence of the energy impressed upon said output circuit.

8. A circuit comprising an electron discharge device having an anode, cathode, control electrode and an auxiliary electrode, energizing means for said device including means for normally maintaining said auxiliary electrode at a higher positive potential with respect to said cathode than said anode, an input circuitincluding said cathode and control electrode and an outp-ut circuit including said anode and cathode, means for impressing signal energy in said input circuit for varying the negative conductance of said output circuit in accordance therewith, the relation between the respective potentials of said anode and auxiliary electrode being such that said output circuit will be normally maintained substantially non-conductive irrespective of signal energy impressed upon said input and means in said output circuit for rendering said output circuit conducting whereby current will iiow therein substantially proportional to said signal energy.

9. In an oscillator `detector circuit an electronic tube having as elements at least an anode, a cathode, a signal grid and an auxiliary grid, means for maintaining both the auxiliary grid and the anode at a positive potential with respect to the cathode, the auxiliary grid being maintained at a substantially higher positive potential than the anode, a circuit including a source of oscillations of one frequency connected between two of the electrodes of said tube, a second circuit including a source of oscillations of a different frequency connected between two of the electrodes of said tube at least one of said last named two electrodes being different than either of said first two named electrodes, one of said tube elements comprising a screening electrode within said tube for screening at least one of the electrodes of said second named two electrodes from at least one of the electrodes of the first named two electrodes.

10. In an oscillator detector circuit, an electronic tube provided with a cathode, and at least three cold electrodes, a source of heating current for said cathode, an energizing potential source for said cold electrodes and arranged so as to maintain two of the cold electrodes at a positive potential with respect to the cathode, one of said last named two electrodes being maintained at a substantially higher positive potential than the anode, a connection including a tuned circuit tuned to one frequency between one of said cold electrodes and the cathode, a connection including a tuned circuit tuned to another frequency between the other of said cold electrodes and said cathode, said third cold electrode being interposed between the other two cold electrodes, to thereby act as a shield between the other two cold electrodes whereby said two circuits are, in effect, isolated from each other.

11. In an oscillator detector circuit an electronic tube provided with a cathode and at least three cold electrodes, a source of heating current for said cathode, an energizing potential source for said cold electrodes, a connection including a tuned circuit tuned to one frequency between one of said cold electrodes and the cathode, a connection including a tuned circuit tuned to another frequency between the other of said cold electrodes and the cathode, said third cold electrode being interposed as a shield between the other two cold electrodes and means for maintaining the interposed cold electrode at a substantially high positive potential with respect to both the other two cold electrodes whereby said two circuits are in effect isolated from one another.

12. In a superheterodyne receiver, a dynatron type frequency changer comprising an electron discharge device provided with a multiplicity of electrodes and connections between various ones of said electrodes for forming input and output circuits for said device, a source of signal voltage, a source of heterodyne voltage, means for coupling one of said Voltage sources to said input circuit and means for coupling the other voltage source to the output circuit, means for energizing the electrodes of the discharge device relative to one another to condition the tube for operation thereof in the vicinity of that point of its plate current-plate potential characteristic curve where the tube remains normally non-conductive despite the application of voltages to the input circuit through the input circuit coupling in the absence of the application of voltages to the output circuit through the output circuit coupling.

WALTER VAN B. ROBERTS. 

